Fix warning reported by Clang static code analyzer:

    CC      blockdev.o
  blockdev.c:2744:5: warning: Value stored to 'ret' is never read
      ret = blk_truncate(blk, size, false, PREALLOC_MODE_OFF, errp);
      ^     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Reported-by: Clang Static Analyzer
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200422133152.16770-3-philmd@redhat.com>
[lv: fix conflict because of "Add flags to blk_truncate()"]
Signed-off-by: Laurent Vivier <laurent@vivier.eu>